Does testosterone create gender bias?

Question

Rodney asked:

Dear Dr Smith

I greatly enjoy listening to you on Friday mornings in Cape Town and am awed by your polymathy.

Some time in the nineties (1998 provides a terminus ad quem because that's when I joined the internet, and I relayed the information to a cyber-friend in America shortly after that) I heard an item on a news bulletin or in a BBC compilation of recent research in Nature to the effect that a study had shown how men with a high level of testosterone have a tendency to produce predominantly female offspring. No biochemical reason was adduced so far as I recall, but the claim was based on a survey of Royal Marine progeny. Perhaps the finding has been discredited since you don't seem to have heard of it, but if it hasn't, it suggests that there IS a way to determine the gender of a child in advance of conception. Marry a Marine if you want a girl, and an interior decorator if you want a boy!

With all good wishes

(Dr) Rodney Stenning Edgecombe

Answer

Ginny - The sex of a baby is determined by whether the egg which is carrying an X-chromosome is fertilised by a sperm carrying another X which will produce a girl or one carrying a Y-chromosome which produces a boy. So, in that sense, itís the sperm that decides the sex of the baby. But what determines which sperm is the first to reach and fertilise that egg or which sperm were even in the mix is a lot more subtle and less well-understood.

Chris - Because the Y-chromosome is likely smaller than the X, some people have argued that they are less of a weight burden for the sperm to push along when it swims and therefore, the Y ones move a bit faster than the X ones. So, depending on when how you time the exposure letís say, you could have a boy or a girl.

Ginny - There is actually a bias, I think itís a 51 to 49 ratio (boys to girls) which fits with that idea that the Y-chromosome sperm is slightly faster, but there are other things that can skew that. Studies seem to show that men with lower levels of testosterone are more likely to produce female offspring and those with high levels are more likely to produce male offspring. So for example, they found that men with prostate cancer which is very strongly linked with high testosterone levels tend to have more male than female children. On the other hand, men who have fertility problems due to lower testosterone tend to produce females when they do father a child. There was even a study conducted at the University of Glasgow that showed that males who ran further than 30 miles a week at the time of conception are much more likely to produce females. If you do a lot of exercise, that tends to temporarily deplete testosterone levels. So, if conception happens relatively soon after the run, then they tend to be more likely to produce females whereas people who were doing just a little bit of running didnít show that difference at all.

Chris - So the bottom line isÖ

Ginny - Low testosterone levels in a male and you're more likely to have girls, but itís very slim differences. Only when you look at hundreds of men proportionally, I donít actually think that going out for a jog beforehand will make you more likely to have a girl.

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